Liquid bottom plate dressings

ABSTRACT

Liquid bottom plate dressings consist of water, a water soluble silicate and an MgO source, the proportion of MgO to silicate being sufficient to convert all the silicate to forsterite (2MgO: SiO2), and the water being sufficient to dissolve all the silicate.

ilnited States Patent Hardy 1 Aug. 28, 1973 [54] LIQUID BOTTOM PLATE DRESSINGS 2,289,681 7/1942 Projahn a a1. 106/3827 2,491,096 12/1949 Feagin 106/3827 lnveflwfl Trevor Hardy, Nechells, England 2,499,729 3/1950 Daussan.... 106/3827 2,521,839 12/1950 Feagin... ll7/5.2 [73] Asslgnee' 3,211,560 l0/l965 Fair 106/3822 g 8 3,184,815 5/1965 Reuter 117/53 [22] Filed: July 22, 1971 [2]] Appl' 165317 Primary Examinerl.orenzo B. Hayes Related US. Application Data Attorney-Wolfe, Hubbard, Leydig, Voit & Osann, [63] Continuation-impart of Ser. No. 29,642, April 17,

1970, abandoned.

[30] Foreign Application Priority Data Dec. 8, 1969 Great Britain 58,922/69 Dec. 8, 1969 Great Britain ..58.923/69 [57] ABSTRACT [52] US. Cl l06/38.27, 106/3823, 1l7/S.3 [51 Int. Cl B28b 7/36 Liquid bottom plate dressings consist of water, a water [58] Field of Search 106/382, 38.22, soluble silicate and an MgO source, the proportion of 106/3823, 38.27, 38.28; ll7/5.l, 5.2, 5.3 MgO to silicate being sufficient to convert all the silicate to forsterite (2MgO:SiO,), and the water being [56] References Cited sufficient to dissolve all the silicate.

UNITED STATES PATENTS 2/1935 Horak 106/3827 7 Claims, No Drawings LIQUID BOTTOM PLATE DRESSINGS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my application Ser. No. 29,642 filed Apr. 17, 1970, now abandoned.

This invention relates to liquid bottom plate dressmgs.

FIELD OF THE INVENTION In the casting of molten metals such as steel to form ingots, molten metal is poured from a height into an ingot mould. Impacting molten metal at the commencement of pouring tends to erode the base areas of the mould, and in order to lower mould wear, it is customary to coat the base and in some cases the lower areas of the walls of the mould, with a refractory dressing. From their common use as coatings on ingot mould bottom plates, these dressings are known as bottom plate dressings, and are for simplicity referred to. as such herein. It is, however, to be understood throughout that their use is not limited to application to bottom plates only, but they may be applied to the bases of bigend-up ingot moulds, and to the walls of ingot moulds, to prevent or minimise molten metal erosion.

DESCRIPTION OF PRIOR ART Many types of dressing have previously been proposed in the past, but as is well known to those engaged in casting ingots, much room for improvement still re mains. Generally, prior types of dressing consist of a particulate refractory such as silica or alumina, a binding agent and a liquid medium, usually water.

GENERAL DESCRIPTION OF INVENTION I have now found that dressings of particular durability and giving enhanced protection may be formed by the use of a liquid coating composition which dries down to a coherent coating on the base or bottom plate of an ingot mould to give a material which, when first contacted by molten metal, reacts to form the highly refractory material Forsterite in situ on the base of the mould.

Forsterite is a highly refractory material of empirical formula 2MgO.SiO which melts at a temperature of about 1,890C in the free state. This is about 200C above the highest steelpoun'ng temperatures. Forsterite occurs naturally in certain mineral materials (e.g., olivine) but I have found that merely using forsterite as a refractory filler does not give the improved protective results obtained when forming the forsterite in situ.

Thus, in order to form forsterite in situ, it is necessary to provide in the bottom plate dressings of this invention a source of MgO and a source of SiO, and tomix the two sources sufficiently intimately to ensure adequate reaction between them to form an effective amount of forsterite. In order to do this, I find it necessary to use as a source of SiO, a water-soluble silicate, and this enables me to produce a liquid dressing in which the SiO, is evenly distributed throughout the MgO, which I use in solid phase.

It is naturally necessary to exercise care in making up the liquid dressings of this invention, in order to ensure that the dressing, before application and drying down on the ingot mould base or bottom plate, will store adquately and will not, for example, gel, react or otherwise deteriorate. Thus, for example, it is not adviseable to use pure Magnesium oxide if the dressing is to be stored for any length of time. Pure MgO hydrates very rapidly and would accordingly be unsuitable for use. Further the source of SiO; should not be one which will react with the MgO source (usually alkaline in character) to form a gel. For this reason, colloidal silica and ethyl silicate do not prove to be satisfactory SiO,

sources.

I thus prefer to use a water soluble alkali metal silicate, such as sodium silicate or potassium silicate, to-

gether with solid MgO, preferably in a form which does not easily hydrate.

I have found that the best results are obtained using calcined or deadburnt magnesite as the source of M g0. Magnestie is a naturally occurring mineral material which always contains a few percent by weight of impurity, but which consists predominantly of MgCO,. On calcination or deadbuming, usually at a temperature of at least l,500C for several hours, CO, is driven off and the calcined or deadburnt material so obtained is hydrateable only to a very limited extent, e.g., to an extent of only up to 3 or 4 percent of its own weight. The best results are obtained from magnesia, extracted from seawater and subsequently dead burnt.

In order to provide an adequately refractory coating on the bottom plate, it is necessary to ensure that all the SiO, is used up in the in situ forsterite formation and that none remains as free silica. This requires a certain minimum ratio of MgOzSiO, in the bottom plate dressing as applied, but it will be appreciated that, since the MgO is in solid granular form, not all of the MgO present in the dressing is available to take part in forsterite formation. The availability of MgO will depend in part on the particle size thereof, being greater as the particle size decreases. Accordingly, I prefer to use MgO of as small a particle size as possible, consistent with the maintenance of stable viscosity, preferably 1 50 or -200 US mesh proved to be best.

If desired, the bottom plate dressings of this invention may contain, in addition to the water soluble silicate and the MgO source, a proportion of a refractory filler which is inert or substantially inert to the first two ingredients. The use of an added inert filler, however, although advantageous in that it serves to lower the cost of the dressing, may have disadvantageous effects. The use of any inert filler may tend to exert a physical masking effect between the silicate and the MgO source, and thus decrease forsterite formation. In addition, there may be a tendency for some slight reaction between the filler and the silicate for example alumina and sodium silicate will react to form an aluminosilicate. If any such reaction occurs, then itis necessary to increase the silicate content of the dressing to maintain forsterite formation at the desired level, though this brings in itsv train the danger of leaving pockets of low-melting silicate in the final coating, which leads to a decrease in the refractoriness thereof.

In general, I have found that the most satisfactory liquid dressings of my invention consist of water, watersoluble alkali metal silicate, and magnesia, calcined magnesite or deadburnt magnesite in finely divided form. The preferred weight proportions lie in the ranges:

water 30 water-soluble alkali metal silicate (as solids) 3 l5 calcined magnesite, dead-burnt magnesite or magnesia 30 60 If an additional inert or substantially inert filler is present; it is preferred that it be present in an amount of at most 30 percent by weight. Fillers such as alumina, zircon, zirconia and titania may be used.

The dressings of this invention are freeely flowing liquids which may be applied to the desired site by any conventional method such as brush, swab or spray. Naturally the thicker the applied coating of dressing, the better the protection, but care should be taken to avoid too thick coatings for reasons of avoiding cracking, as well as those of economy. Generally, a preferred coating thickness, when the coating has dried to a layer of the solids ingredients containing at most only a few percent by weight of entrapped water, is 0.5 to 5 mm.

in use of the coatings, the first time molten metal is poured onto the coating, the forsterite-forming reaction takes place, and results in a refractory layer containing particles of MgO and of any inert refractory filler originally present, bonded together and to the ingot mould base of bottom plate, by a matrix of forsterite. The layer is free of low-melting material such as silica, and thus forms a highly resistant and protective coating. It is found in practice, that this coating will resist molten metal erosion for several casting cycles after its original application.

SPECIFIC EXAMPLES OF THE INVENTION EXAMPLE 1 A bottom plate dressing was made up of (proportions by weight): Magnesia (deadbumt magnesite; 100% less than 0.075 mm) 33.75% Alumina (all less than 0.15 mm) 11.25% Liquid sodium silicate (SiO,:Na,O ratio 3.3:1,

37.9% solids, 8.6.1.39) 12.00% Suspending agent (Xanthomonas colloid:

Biopolymer XB 23, Melle-Bezons) 0.15% Water 42.85%

EXAMPLE 2 A bottom plate dressing was made up of (proportions by weight): Magnesia (deadburnt magnesite; 100% less than 0.075mm) 45% Sodium metasilicate .5H,0 (35% by weight aqueous solution) 22.75% suspending agent (As example 1) 0.15% water 32.1%

This was sprayed onto a heated cast iron slab (as used in Example 1) and allowed to dry. Coating thickness was 2 mm. Thereafter the cast iron slab was arranged with the coated face uppermost and in a plane at an angle of 45 to the horizontal. A stream of molten steel at 1,620C was then poured onto the slab from a height of 45 cm at a rate of 2 kg of steel per second. After 90 Results were area of photo micrograph):

Periclase (M30) 35% Forsterite 50% Spinels, glasses and other silicates 15% X-ray diffraction analysis of a quantity of the powdered coating residue indicated an overall Forsterite content, taken over the total coating thickness, of 6 to 8 percent.

I claim as my invention 1. A bottom plate dressing consisting essentially of water, a water soluble silicate and a source of MgO selected from the group consisting of calcined magnesite and dead-burnt magnesite, said magnesite being fired at a temperature of a least l,500C for a time sufficient to render it hydratable up to a maximum of 4 percent of its own weight, the relative proportions of MgO and silicate being such that enough MgO is available for reaction with all the SiO, present to form forsterite (2MgO.SiO and the amount of water present being sufficient wholly to dissolve the water-soluble silicate.

2. A bottom plate dressing according to claim 1 wherein the relative weight proportions of the ingredients are in the following ranges water 30 MgO source 30 60 water-soluble silicate 3 15 3. A bottom plate dressing according to claim 1 wherein the water soluble silicate is selected from the class consisting of water soluble sodium silicates and water soluble potassium silicates.

4. A bottom plate dressing according to claim 2 which contains up to 30 percent by weight of a refractory filler selected from the class consisting of zircon, zirconia, titania and alumina.

5. In the process of protecting ingot mould bases and bottom plates from erosion by molten metal, the improvement which comprises applying to the surface a liquid coating composition consisting essentially of water, a water soluble silicate and a source of MgO selected from the group consisting of calcined magnesite and dead-burnt magnesite, said magnesite being fired at a temperature of at least 1,500C for a time sufficient to render it hydratable up to a maximum of 4 percent of its own weight, the relative proportions of MgO and silicate being such that enough MgO is available for reaction with all the Si0 present to form forsterite (2MgO.SiO,), and the amount of water present being sufficient wholly to dissolve the water-soluble silicate, and subsequently drying the applied coating.

6. The process of claim 5 wherein the substantially dry coating has a thickness of 0.5 to 5 mm.

7. The process of claim 5 wherein the applied coating is heated by pouring molten metal onto the coating.

i i i 4' i 

2. A bottom plate dressing according to claim 1 wherein the relative weight proportions of the ingredients are in the following ranges water 30 - 70 % MgO source 30 - 60 % water-soluble silicate 3 -15 %
 3. A bottom plate dressing according to claim 1 wherein the water soluble silicate is selected from the class consisting of water soluble sodium silicates and water soluble potassium silicates.
 4. A bottom plate dressing according to claim 2 which contains up to 30 percent by weight of a refractory filler selected from the class consisting of zircon, zirconia, titania and alumina.
 5. In the process of protecting ingot mould bases and bottom plates from erosion by molten metal, the improvement which comprises applying to the surface a liquid coating composition consisting essentially of water, a water soluble silicate and a source of MgO selected from the group consisting of calcined magnesite and dead-burnt magnesite, said magnesite being fired at a temperature of at least 1,500*C for a time sufficient to render it hydratable up to a maximum of 4 percent of its own weight, the relative proportions of MgO and silicate being such that enough MgO is available for reaction with all the SiO2 present to form forsterite (2MgO.SiO2), and the amount of water present being sufficient wholly to dissolve the water-soluble silicate, and subsequently drying the applied coating.
 6. The process of claim 5 wherein the substantially dry coating has a thickness of 0.5 to 5 mm.
 7. The process of claim 5 wherein the applied coating is heated by pouring molten metal onto the coating. 